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ORAUT Responses to 

Fernald SC&A 

Clarifying Questions

Response Paper

National Institute for Occupational 

Safety and Health

March 13, 2018

Stuart Hinnefeld, Director

Division of Compensation Analysis and Support

This is a working document prepared by NIOSH’s Division of Compensation Analysis and Support (DCAS) or its contractor for use in discussions 

with the ABRWH or its Working Groups or Subcommittees. Draft, preliminary, interim, and White Paper documents are not final NIOSH or 

ABRWH (or their technical support and review contractors) positions unless specifically marked as such. This document represents preliminary 

positions taken on technical issues prepared by NIOSH or its contractor. NOTICE: This report has been reviewed to identify and redact any 

information that is protected by the Privacy Act 5 USC §552a and has been cleared for distribution.
Response Paper ORAUT Responses to Fernald SCA Clarifying Questions March 13, 2018

Page 2 of 6 

This is a working document prepared by NIOSH’s Division of Compensation Analysis and Support (DCAS) or its contractor for use in discussions 

with the ABRWH or its Working Groups or Subcommittees. Draft, preliminary, interim, and White Paper documents are not final NIOSH or 

ABRWH (or their technical support and review contractors) positions unless specifically marked as such. This document represents preliminary 

positions taken on technical issues prepared by NIOSH or its contractor. NOTICE: This report has been reviewed to identify and redact any 

information that is protected by the Privacy Act 5 USC §552a and has been cleared for distribution.

RECYCLED URANIUM

SC&A Clarifying Question 2: How was NIOSH able to determine which lots were processed 

prior to 1973 as was indicated in Figure 1 (above) as well as the most recent responses contained 

in NIOSH 2017b? 

NIOSH’s current approach in ORAUT-TKBS-0017-5, the Feed Materials Production Center – 

Occupational Internal Dose Technical Basis Document used the information found in the Ohio 

Field Office Report (SRDB Ref ID #3644). The FEMP Lot Marking and Color Coding System 

Requirements Manual Number RM-0005 (Pages 678-735 of SRDB Ref ID #3644) can be used to 

identify when a lot was processed. This is covered in Section 2.5 of the Ohio Field Office report 

(i.e. Lot or Sequence Number). This section indicates that one can determine the month of 

production for a given lot by converting the Sequence Number to a date. This is how NIOSH 

determined which lots in Attachment C were processed prior to 1973. 

SC&A Clarifying Question 3: NIOSH 2011 indicates that for the period prior to 1973, Subgroup 

6A (UO3 from Hanford) was utilized in arriving at 10 Pu ppb, though it is not clear if this 

assumption is still being utilized. It is also not clear if the Subgroup 6A data was extrapolated 

backwards from post-1973 to the earlier period. If data is being back extrapolated, further 

discussion of the use of this later data as a substitute may be warranted.

NIOSH is no longer using the determination on the analysis performed in NIOSH 2011, Issues 

Related to the Ability to Bound Internal Dose from Recycled Uranium Trace Level 

Contaminants at Fernald, Draft 01. NIOSH’s use of the 10 ppb Pu in recycled uranium (RU) 

through 1972 is based on the lot data found in Attachment C of the Ohio Field Office Report 

(SRDB Ref ID #3644). Based on a qualitative review of the lots identified as being processed 

prior to 1973, it appears that more than 95% of the identified Pu concentrations are below 1 ppb. 

Of those that are above 1 ppb Pu, most of those are still below 10 ppb Pu. This means that any 

potential to exposures to RU with Pu concentrations greater than 10 ppb would be considered 

rare and to be short in duration. The remaining, and thus, majority, of any worker’s exposure 

time would have been associated with materials well below the recommended default level of 10 

ppb Pu. Therefore, NIOSH cannot identify any reason to believe that any worker could have an 

exposure for which the recommended default level of 10 ppb Pu would not be adequately 

conservative and bounding. 

SC&A Clarifying Question 4: What analytical basis was used to determine the potential 

contaminant ratios experienced by workers involved in the MgF process in the 1961-1972 

period? Furthermore, what is the analytical basis used to conclude that workers exposed to the 

MgF source term would be sufficiently “averaged out” with exposures to other, less 

contaminated, recycled uranium?

Similar to the response for Clarifying Questions 3 above, NIOSH is no longer using the 

determination on the analysis performed in NIOSH 2011, Issues Related to the Ability to Bound 

Internal Dose from Recycled Uranium Trace Level Contaminants at Fernald, Draft 01. NIOSH’s 

use of the 10 ppb Pu in RU through 1972 is based on the lot data found in Attachment C of the 
Response Paper ORAUT Responses to Fernald SCA Clarifying Questions March 13, 2018

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This is a working document prepared by NIOSH’s Division of Compensation Analysis and Support (DCAS) or its contractor for use in discussions 

with the ABRWH or its Working Groups or Subcommittees. Draft, preliminary, interim, and White Paper documents are not final NIOSH or 

ABRWH (or their technical support and review contractors) positions unless specifically marked as such. This document represents preliminary 

positions taken on technical issues prepared by NIOSH or its contractor. NOTICE: This report has been reviewed to identify and redact any 

information that is protected by the Privacy Act 5 USC §552a and has been cleared for distribution.

Ohio Field Office Report (SRDB Ref ID #3644). Based on a qualitative review of the lots 

identified as being processed prior to 1973, it appears that more than 95% of the identified Pu 

concentrations are below 1 ppb. Of those that are above 1 ppb Pu, most of those are still below 

10 ppb Pu. This means that any potential to exposures to RU with Pu concentrations greater than 

10 ppb would be considered rare and to be short in duration. The remaining, and thus, majority, 

of any worker’s exposure time would have been associated with materials well below the 

recommended default level of 10 ppb Pu. Therefore, NIOSH cannot identify any reason to 

believe that any worker could have an exposure for which the recommended default level of 10 

ppb Pu would not be adequately conservative and bounding. 

Additionally, some insight about “averaging out” the higher than typical TRU:U ratios in MgF 

can be obtained from a site expert interview, SRDB 38132. In lines 244 through 251 of that 

interview the expert relates that metal production (which generated the MgF) and metal 

fabrication generated residues that were sent back through the refinery to reclaim the enriched U.

The total residues reclaimed in that fashion were about 3% to 5% of the enriched uranium 

produced. This means that over 90% of the enriched material processed in a year came from 

sources other than Fernald residues, so the TRU:U ratio found in MgF would be averaged out by 

the other sources.

THORON

SC&A Clarifying Question 5: What additional resources, references or documentation (if any) 

were consulted in determining that the originally assumed release fraction of 10-4 is the most 

appropriate? 

A release fraction is inherently an empirical simplification of the process of emanation, 

diffusion, advection, absorption and adsorption, which were discussed by Rogers and Nielson in 

SRDB Ref ID #27376. With so many variables and no specific data available to individually 

quantify each one, empirical experience provides the best available basis for selection of the 

release fraction parameter. No other references or documentation for the selection of the value 

was used. As noted in the discussion above, the release fraction is expected to range from 10-3 

to 10-4, (in other words, from 0.1% to 0.01%). When this value is assumed, the thoron exposure 

calculated in Clarifying Question 7 item #1 (below) matches well with the empirically observed 

thoron exposure shown in Clarifying Question 7 item #2 (below). 

SC&A Clarifying Question 6: NIOSH 2017b indicates that the occupancy factor should remain 

at 25% (3 months per year) for the period from 1954 to 1989 but does not provide any additional 

justification or discussion. It is also not clear if the assumption of 1 month of exposure per year 

from 1990 to 2006 still applies. SC&A requests clarification as to how the currently assumed 

occupancy times were substantiated.

These occupancy factors are based on the assumption that storage facilities are not occupied full 

time as processing facilities would be. The occupancy factor could range from zero to 100%, 

both of which seem unlikely for a storage facility. In Building 65, the assumed value, along with 

the assumed release rate, resulted in a plausible agreement with a measured value. The 1 month 
Response Paper ORAUT Responses to Fernald SCA Clarifying Questions March 13, 2018

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This is a working document prepared by NIOSH’s Division of Compensation Analysis and Support (DCAS) or its contractor for use in discussions 

with the ABRWH or its Working Groups or Subcommittees. Draft, preliminary, interim, and White Paper documents are not final NIOSH or 

ABRWH (or their technical support and review contractors) positions unless specifically marked as such. This document represents preliminary 

positions taken on technical issues prepared by NIOSH or its contractor. NOTICE: This report has been reviewed to identify and redact any 

information that is protected by the Privacy Act 5 USC §552a and has been cleared for distribution.

of exposure per year from 1990 to 2006 assumption of exposure per year from 1990 to 2006 does 

not apply, since the three-month value in NIOSH 2017b, Issues Resolution Matrix for Fernald 

Site Profile and SEC Petition, Rev. 05, is more conservative.

SC&A Clarifying Question 7: It is not clear what the final thoron dose assignment is intended to 

be, as NIOSH 2017a and NIOSH 2017b contain different estimates. In addition, SC&A asks for 

clarification on the calculation steps presented in NIOSH 2017b, as it appears the conversion 

from working levels to working level-months in NIOSH 2017b may be in error. 

The calculated thoron dose in NIOSH 2017b, Issues Resolution Matrix for Fernald Site Profile 

and SEC Petition, Rev. 05, is meant to replace the estimates provided in NIOSH 2017a, 

ORAUT-TKBS-0017-5, Feed Materials Production Center – Occupational Internal Dose 

Technical Basis Document. The recommendation is to assign thoron doses to radiological 

workers for the entire period of potential exposure, 1954-2006. This is based on a calculation 

from October 2017, which uses the highest thoron concentration value from Building 65. 

This equation is used to calculate the equilibrium concentration of Rn-220 and progeny. 

This is used to adjust the equilibrium, apply the occupancy factor and convert from pCi/L to WL. 

The highest FMPC thoron exposures are likely to be associated with storage facilities because 

the mass of material in storage exceeds the amount in process daily. Large scale storage 

operations are typically occupied only a fraction of the work day. Ventilation in storage facilities 

is likely to be at a lower air change rate when compared to ventilation in a processing facility 

where vigorous mechanical or chemical processes are conducted. ORAUT-TBKS-017-5, Rev 3, 

Table B-5, “Summary of thoron exposure estimates”, shows, in the last two rows, various storage 

sites throughout the site’s history contained as much as 300 MT (300*106 g) of thorium. Using 

a value of 0.989*105 pCi/g, there could be 3*1013 pCi of Th-232 in storage. 

The thorium in storage was chemically purified at some time prior to storage. That created a 

disequilibrium between Th-232 activity and Th-228 activity, which would be a variable as a 

function of time, initially decreasing and then increasing several years later. The time since 

chemical separation is not known, but assuming only one chemical separation occurred, the 

disequilibrium cannot be less than 65%, or more than 100%. Since 100% is a bounding 

assumption, that value is selected. It follows that the bounding activity of Th-228 in storage, and 

its short-lived the decay product, thoron, will be 3*1013 pCi. 
Response Paper ORAUT Responses to Fernald SCA Clarifying Questions March 13, 2018

Page 5 of 6

This is a working document prepared by NIOSH’s Division of Compensation Analysis and Support (DCAS) or its contractor for use in discussions 

with the ABRWH or its Working Groups or Subcommittees. Draft, preliminary, interim, and White Paper documents are not final NIOSH or 

ABRWH (or their technical support and review contractors) positions unless specifically marked as such. This document represents preliminary 

positions taken on technical issues prepared by NIOSH or its contractor. NOTICE: This report has been reviewed to identify and redact any 

information that is protected by the Privacy Act 5 USC §552a and has been cleared for distribution.

A release fraction for the thoron in storage must be assumed. Release of the inert noble gas 

thoron under storage conditions occurs by a diffusion process. The short half-life of thoron (56 

s) limits the amount of diffusion that can occur before the element loses its noble gas property 

and diffusion stops. This means that release fractions for material in storage tend to be small. 

Assuming a relatively large release fraction value of 1*10-4, the airborne thoron activity in a 

large storage facility is 3*109 pCi. The estimated volume of the airspace in an FMPC storage 

building is 1*107 L, so the thoron air concentration is 300 pCi/L.

The Potential Alpha Energy Concentration (PAEC) described in Table B-3 shows that there is 

1.33 x 105 MeV/L per 7.5 pCi/L of thoron. PAEC can be expressed in Working Level, where 

7.5 pCi of thoron in equilibrium with the short-lived decay products equals one Working Level. 

A 300 pCi/L concentration of thoron in equilibrium with its short-lived decay products is 40 WL.

In a 2006 paper, Harley and Chittaporn reported that the indoor thoron equilibrium factor, Feq, is 

likely to be 0.02 and cannot exceed 0.04. Feq describes the activity relationship between thoron 

and its short-lived decay products which contribute to respiratory tract dose. Feq can be used as 

a factor to modify Working Level. Using a value of Feq of 0.04, the Working Level is 1.6. 

Assuming an occupancy factor in the storage facility of 25% (three work months per work year) 

the thoron exposure potential is 0.4 WLM per year.

Additionally, the following information provides a better explanation of how default values for 

thoron exposure in Building 65 is estimated:

A thoron concentration of 267 pCi/L was measured in Building 65 during the first calendar 

quarter of 1996, as reported in Renk (SRDB Ref ID # 133254, p.65), using a track-etch device. 

This was by far the highest reported result in that large data set. Although it is an outlier in the 

context of the average measurement for that period at Fernald, it probably is an accurate 

indicator of the thoron concentration in that building.

The Potential Alpha Energy Concentration (PAEC) described in Table B-3 shows that there is 

1.33*105 MeV/L per 7.5 pCi/L of thoron. PAEC can be expressed in Working Level, where 7.5 

pCi of thoron in equilibrium with the short-lived decay products equals one Working Level. A 

267 pCi/L concentration of thoron in equilibrium with its short-lived decay products is 35.6 WL.

In a 2006 paper, Harley and Chittaporn reported that the indoor thoron equilibrium factor, Feq, is 

likely to be 0.02 and cannot exceed 0.04. Feq describes the activity relationship between thoron 

and its short-lived decay products which contribute to respiratory tract dose. Feq can be used as 

a factor to modify Working Level. Using a value of Feq of 0.04, the Working Level is 1.42. 

Assuming an occupancy factor in the storage facility of 25% (three work months per work year),

the thoron exposure potential is 0.36 WLM per year.

Considering that this measured value represented only one calendar quarter out of many, it is 

unlikely that this is a bounding analysis of the thoron exposure potential in Building 65. A 

bounding analysis might be expected by increasing the thoron exposure by 50%. This results in 

a rounded value of 0.5 WLM per year.
Response Paper ORAUT Responses to Fernald SCA Clarifying Questions March 13, 2018

Page 6 of 6

This is a working document prepared by NIOSH’s Division of Compensation Analysis and Support (DCAS) or its contractor for use in discussions 

with the ABRWH or its Working Groups or Subcommittees. Draft, preliminary, interim, and White Paper documents are not final NIOSH or 

ABRWH (or their technical support and review contractors) positions unless specifically marked as such. This document represents preliminary 

positions taken on technical issues prepared by NIOSH or its contractor. NOTICE: This report has been reviewed to identify and redact any 

information that is protected by the Privacy Act 5 USC §552a and has been cleared for distribution.

The value used to describe the disequilibrium between Th-232 and Th-228 could be any 

number less than 100%. Triple-separated thorium, with its equilibrium value of 23%, was 

selected for use when interpreting in vivo lung counting data, because, in that context, a lower 

value is more favorable to the claimant than a higher value. In the context of thoron exposure 

based on air concentration of radioactive material, a higher equilibrium value is more favorable. 

For this calculation, a value of 100% is chosen as a bounding and claimant-favorable 

assumption.

The value, Feq, used to describe the equilibrium between thoron (Rn-220) and its shortlived decay products, is used to modify the Working Level calculated with the assumption of 

100% equilibrium. Values of Feq could range from less than 1% to 100%. Higher thoron 

exposures result from higher values of Feq. Empirically, Feq has been observed to be very low, 

only a few percent. In a 2006 paper, Harley and Chittaporn reported on the indoor thoron 

equilibrium factor and included data from Fernald in the analysis. They concluded that 0.04 was 

a bounding value.

A release fraction for the thoron from solid thorium metal or sol-gel must be assumed. 

Release of the inert noble gas, thoron, under storage conditions occurs by a diffusion process. 

The short half-life of thoron (56 s) limits the amount of diffusion that can occur before the 

element loses its noble gas property and diffusion stops. This means that release fractions for 

material in storage tend to be small. We believe that a bounding value for this factor is 1*10-4. 

When this value is assumed, the thoron exposure calculated in item 1 (above) matches well with 

the empirically observed thoron exposure shown in item 2 (above).

The thoron exposure for the “Passive storage” period, 1986 through 1987, will be 

changed to match the thoron exposure assigned during the “Long-term DOE repository storage” 

period.

In summary, a bounding analysis might be expected by increasing the thoron exposure by 50%. 

This results in a rounded value of 0.5 WLM per year. This can be assigned to all radiological 

workers from 1954-2006, and for all locations.